This invention relates to digital controllers, and more particularly to a direct digital controller (DDC) in which isolation means is provided between an analog signal circuit and a digital circuit.
A direct digital controller produces a direct current signal (4 to 20 mA (milliampere) which represents a manipulated variable signal and which is applied to an acutator such as a motor valve. A block diagram representative of a DDC is shown in FIG. 1. The DDC, as shown in FIG. 1, can be used to control a motor valve in a process.
Analog signals from a plurality of detectors 10A, 10B, . . . are applied to an analog input circuit 12 which comprises a multiplexer 14 and an analog-to-digital (A/D) converter 16. The analog signals are scanned in a time-sharing manner. Each time an analog signal is scanned the signal is converted to a corresponding digital signal by the A/D converter 16. The digital signal is provided as an input of a central processor unit (CPU) 18 which comprises, for example, a microprocessor. The digital signal received by the CPU 18 is processed and stored in a ten-bit up-down counter 20.
A predetermined value corresponding to a base or reference operation of an actuator 22 is initially stored in the up-down counter 20 by the CPU 18. When the operation of the actuator 22 changes, the CPU 18 provides an appropriate up-count signal or down-count signal to the up-down counter 20 as an error signal from the base or reference value. An instruction signal corresponding to a manipulated variable signal which is applied to the actuator 22 is digitally stored in the up-down counter 20.
The instruction signal stored in the up-down counter 20 is converted to an analog manipulation voltage signal AV by a digital-to-analog (D/A) converter 24. The voltage signal AV is converted to an analog manipulation current signal AI by a voltage-to-current (V/I) converter 26.
The current signal AI is applied to the motor valve actuator 22 as the manipulated variable signal. The DC voltage value of the manipulation voltage signal AV varies within the range between 0 to 5 V in accordance with the opening of the motor valve 22. The DC current value of the current signal AI also varies within the range between 4 to 20 mA.
The manipulation voltage signal AV of the D/A converter 24 is supplied as a feedback signal to the multiplexer 14 for the purpose of correcting the errors in the signal values in the digital processing loop including the CPU 18.
In the DDC shown in FIG. 1, the signal transfer lines connected between the CPU 18 and the detectors 10A, 10B, . . . and the signal line to the motor valve 22 are oftentimes interconnected with other process equipment located at great distances from the DDC. These signal lines are often plagued with noise signals such as common mode voltage signals or impulse noise signals. The output voltage signal AV of the D/A converter 24 has a potential level applied to the multiplexer 14 as the feedback signal. However, the output of the V/I converter 26 cannot be examined by CPU 18.
FIG. 2 shows a schematic block diagram of a prior art DDC. In the DDC shown in FIG. 2, the analog input circuit and the CPU 18 are connected by an isolation transformer 28. Similarly, the D/A converter 24 and the V/I converter 26 are connected by an isolation transformer 30. Manipulation current for the motor valve 22 is detected by another isolation transformer 32 and the manipulation current is applied to the multiplexer 14 as a feedback signal. In the DDC shown in FIG. 2, however, the analog signal loop and the digital signal loop must be isolated by the isolation transformers. Since the analog signal such as the manipulation current for the motor valve must be used as the feedback signal, there is still a problem that noise signals will be present in the analog signal.